There are a number of existing proposals for quantum computing (QC) based on charge qubits in the solid-state. There is also the Kane proposal for a real spin quantum device based upon phosphorus donor atoms in silicon. Proposals relevant to this invention are now presented in chronological order:    Logical devices implemented using quantum cellular automata, P. D. Tougaw and C. S. Lent, J. Appl. Phys. 75, 1818 (1994).    Conditional quantum dynamics and logic gates, A. Barenco, D. Deutsch, Ekert and Josza, Phys. Rev. Lett. 74, 4083 (1995).    Minimal energy requirements in communication, R. Landauer, Science 272, 1914 (1996).    Coupled quantum dots as quantum XOR gate, J. A. Brum and P. Hawrylak, Superlattices and Microstr. 22, 431 (1997).    Quantum information in semiconductors: Noiseless encoding in a quantum-dot array, P. Zanardi and P. Rossi, Phys. Rev. Lett. 81, 4752 (1998).    Quantum computation with quantum dots, D. Loss and D. P. DiVincenzo, Phys. Rev. A 57, 120 (1998).    A silicon-based nuclear spin quantum computer, B. E. Kane, Nature 393, 133 (1998).    Microwave spectroscopy of a quantum-dot molecule, T. H. Oosterkamp, T. Fujisawa, W. G. van der Wiel, K. Ishibashi, R. V. Hijman, S. Tarucha and L. P. Kouwenhoven, Nature 395, 873 (1998).    Dephasing in open quantum dots, A. G. Huibers, M. Switkes, C. M. Marcus, K. Campman and A. C. Gossard, Phys. Rev. Lett. 81, 200 (1998).    Coupled quantum dots as quantum gates, G. Burkard, D. Loss and D. P. DiVincenzo, Phys. Rev. B 59, 2070 (1999).    Quantum gates by coupled asymmetric quantum dots and CNOT-gate operation, T. Tanamoto, Phys. Rev. A 61, 022305 (2000).    Coherent properties of a two-level system based on a quantum-dot photodiode, A. Zrenner, E. Beham, S. Stufler, F. Findeis, M. Bichler and G. Abstrieter, Nature 418, 612 (2002).    High coherent solid-state qubit from a pair of quantum dots, Xin-Qi Li and YiJing Yan, quant-ph/0204027 (2002).    Two-electron quantum dots as scalable qubits, J. H. Jefferson, M. Fearn, D. L. J. Tipton and T. P. Spiller, quant-ph/0206075 (2002).
The potential of quantum information processing relies on one of the most fragile resources in nature—quantum entanglement. Even without the destructive influence of environmental decoherence a practical quantum computer requires hundreds, if not thousands of gates to perform useful tasks. Since a system of qubits can never be totally isolated from the environment, error-correcting schemes must be incorporated in the design and encoding of the qubits, further increasing the number of qubits required.
The real challenge is to prove the principle of QC in a technology which is inherently scalable. This does not simply mean the ability to make a large number of interconnected qubits. The concept of scalability requires that the error probability per qubit be kept below some threshold determined by fault tolerant implementations. What that threshold actually is depends on the particular implementation and the physical origin of decoherence.